Polyurethanes come to mind first when one thinks of foam products, and indeed polyurethanes dominate the solid foam market. Such foams may be either rigid or flexible, depending on how the manufacture takes place. In fact, polyurethane systems allow enormous variations in the polymerization and fabrication processes; it is this complexity which keeps the urethane area a fertile field for development and expansion.
Like many macromolecules, polyurethanes are a general class of materials which can be prepared via many different routes--at least in principle. However, industrial practicalities dictate a preferred approach based on feedstock availability, ease of processing, etc. For example, ordinary condensation polymerization of bischloroformates with diamines will yield polyurethanes, but the universal large scale practice calls for condensation of diisocyanates with diols. (More generally the common synthesis involves diisocyanates and polyols--of which the diol is a special case--where triol species produce crosslinking.) A typical instance might have 2,4-toluene diisocyanate (TDI) reacting with 1,4-butanediol. In any case, the practical problems show up not at the level of individual chemical molecules but rather with the physical production and molding steps.
Polyurethanes are notoriously defiant regarding fabrication. The production of a good, useful foam object involves precise control over the size and distribution of the hollow voids, or cells in the product. An open cell foam would make a poor life preserver while a closed cell foam would make a poor sponge. Volumes have been written on the problems associated with polyurethane processing, and the subject is generally beyond the scope of this discussion, except as relates to prepolymer stabilization.
Most polyurethanes cannot simply be made into a melt and injected into a mold in the way that polyethylene normally perform. One viable method is the "one shot" approach, whereby all the reactants are combined simultaneously with injection into the mold. The alternative process calls for controlled synthesis of a prepolymer, i.e., a short chain polyurethane intermediate. The use of the intermediate provides a polyurethane which has generally better properties. The prepolymer method is generally more forgiving than the one shot approach, and hybrid techniques are possible, but the present art still has much room for improvements. This patent addresses the practical problem of prepolymer stability.
In particular, the present invention provides for treatment of polyols: this process involves the treatment of polyalkylene carbonate polyols, leading to more stable prepolymers and improved urethane products. Polyalkylene carbonate (PAC) polyols may be made by a base-catalyzed reaction, and some catalyst remains in the product PAC. Accordingly, the prior art has depended on residual acid species, e.g., HCl, in the TDI to neutralize the residual base species in the polyol. Where necessary, it is possible to add an acid chloride to the TD1 (invariably benzoyl chloride) to provide for the neutralization; but the limitation on the prior art is that benzoyl chloride simply does not stabilize PAC prepolymers--even when added in large excess. Benzoyl chloride may prevent a runaway exothermic reaction, but even so, it is just as objectionable as HCl for many applications because residual chloride ions remain in the product. Even further, benzoyl chloride does not provide a stable prepolymer. The present invention produces stable PAC prepolymers with the dual advantages of longer storage times (before fabrication) and longer gel times (during fabrication). Thus, premature curing does not occur, and the molded products have better physical properties, environmental resistance, etc.